I read Wikipedia a bit more and apparently the French occupation of the Rhine was actually quite profitable:
"According to Sally Marks, the occupation of the Ruhr 'was profitable and caused neither the German hyperinflation, which began in 1922 and ballooned because of German responses to the Ruhr occupation, nor the franc's 1924 collapse, which arose from French financial practices and the evaporation of reparations'. Marks suggests the profits, after Ruhr-Rhineland occupation costs, were nearly 900 million gold marks.
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If this is true then I think it's mostly down to whether or not France has the political will to do so. Which is questionable, but I'm sure there's some PoD that could lead to such will being present.
Unrelated to that, the assertion that a French-backed Rhenish Republic would become a European Vietnam seems questionable to me. It's not like Germany was in a particularly good position at this point either, so as long as France lets the Rhine stay relatively economically and politically independent I don't see where the political will to destroy developed, well-off European cities just to get the French out comes from. As per usual I might just be totally wrong though.